Color CRTs as used in television receivers and computer terminals employ multiple electron beams for providing a video image comprised of the primary colors of red, green and blue on a display screen. Typically, three electron beams are directed onto and scan the display screen in a raster-like manner. A common color CRT design has the three electron beams arranged in an inline array. An electron gun in the color CRT typically employs plural cathodes for generating energetic electrons in combination with apertured, charged grids aligned along the electron gun's longitudinal axis. Each of the charged grids typically includes three inline beam passing apertures for forming the energetic electrons into three separate beams and for focussing the beams on the CRT's display screen.
In order to provide a high quality color video image on the CRT's display screen, each of the gun's charged grids must be in precise position and alignment along the electron gun's longitudinal axis. An alignment jig incorporating three spaced mandrels is typically used for positioning and aligning the grids during electron gun assembly. Where the three inline electron beam passing apertures are circular, cylindrically shaped mandrels are employed for engaging those portions of the grid defining the three inline apertures therein. The cylindrically shaped mandrels provide the requisite alignment precision of the grids necessary for electron gun assembly.
However, in some cases non-circular beam passing apertures are provided in the electron gun grid for improved video image quality. For example, the two outer electron beam passing apertures may be provided with an asymmetric, e.g., eccentric, shape to compensate for the asymmetric electrostatic field applied to the two outer electron beams. This asymmetric electrostatic field arises from the inline alignment of the three electron beams and the shape of the common lens portions of grids in the electron gun's main focus lens. The use of a cylindrical mandrel inserted in an asymmetric beam passing aperture typically gives rise to rotation of the grid during electron gun assembly. Rotation of the grid misaligns the grid's electron beam passing apertures resulting in a degradation of video image quality presented on the CRT's display screen.
The present invention addresses the aforementioned limitations of the prior art by providing for the precise alignment of grids having asymmetric beam passing apertures during electron gun assembly. The present invention also provides apparatus for the stable and secure positioning of electron gun grids during electron gun assembly.